1. Field of the Invention
The present invention relates to a novel acoustic wave sensor for sensing acoustic waves in a fluid typically air. The invention employs optical techniques, and more particularly depends upon the phase modulation which occurs when coherent light is passed through a fluid in which acoustic waves occur. The phase modulation may be imposed on a carrier provided by optical heterodyning and then detected electrically. The electrical signal recovered in this manner is representative of the acoustic waves.
2. Description of the Prior Art
In the patent application of Gordon Jacobs, Ser. No. 507,528 filed Sep. 19, 1974 entitled xe2x80x9cLaser Hydrophone and Virtual Array of Laser Hydrophonesxe2x80x9d, an acoustic sensor employing optical techniques was earlier proposed. The sensor, which was termed a xe2x80x9chydrophonexe2x80x9d, since it was designed for use in water, employed a laser beam which was focused on a small xe2x80x9cfocalxe2x80x9d volume of water in which natural light scattering matter was suspended. The scattering matter, which vibrates in synchronism with any acoustic waves present, produces a phase modulation of the scattered light. The phase modulation was then recovered by optical heterodyne and sensitive phase demodulation techniques. The Jacobs arrangement contemplated both single hydrophones and arrays of hydrophones. In general, optical arrays, such as the Jacobs arrays, produce less hydrodynamic disturbance than the known large area piezoelectric arrays.
The Jacob""s arrangement was dependent upon a xe2x80x9cdopplerxe2x80x9d type shift in frequency or phase, the doppler shift being caused by particle motion, toward and away from the sensing beam. Thus the maximum sensitivity was obtained by pointing the laser beam in a direction toward the source of acoustic waves and perpendicular to the wave fronts.
The present invention shares certain of the objectives and in using optical techniques, certain of the means of the foregoing Jacobs"" invention.
The Jacobs invention is unlike the present invention which has application to the detection of acoustic waves in the air. The present invention has application to security systems, as for instance in the monitoring of sounds occurring along the perimeter of a property.
Accordingly it is an object of the present invention to provide an improved sensor of acoustic waves occurring in a fluid medium.
It is another object of the present invention to provide an improved optical acoustic sensor for sensing air borne acoustic waves.
These and other objects of the invention are achieved in an optical acoustic sensor comprising means for producing mutually coherent optical sampling and reference beams; optical means including an aperture at which the path of the sampling beam into the air is initiated and after reflection is terminated, and light reflective means arranged in the path of the sampling beam for reflecting significant sampling beam energy back via the aperture.
The optical acoustic sensor further comprises an optical detector for coherently combining the reflected sampling beam with the reference beam to form an electrical heterodyne signal, phase modulated as result of the acoustic wave induced variation in the index of refraction, and finally a phase detector coupled to the output of the optical detector for detecting the acoustic wave induced phase variation of the sampling beam and thereby recovering an electrical signal representative of the acoustic waves.
In accordance with the invention, the initial and reflected portions of the sampling beam path between aperture and the light reflective means are oriented with a substantial component parallel to the acoustic wavefronts of the acoustic waves. For maximum sensitivity, the sampling beam path is parallel in the farfield or tangential in the nearfield to the acoustic wave fronts. With a substantial component parallel to the acoustic wave front, the sampling beam is exposed to an acoustic wave induced density variation of like amplitude over a portion of the path of the sampling beam. As the acoustic waves traverse the path of the beam of light, the density variation produced by the acoustic wave, causes the index of refraction of the fluid (air) to vary, and thereupon phase modulates.the sampling beam. The amount of phase modulation is in proportion to the accumulated variation in the index of refraction over the beam path.